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Substrates and their U-Values

The ability of a building to maintain a comfortable temperature and reduce energy consumption largely depends on the insulating properties of the materials used in its construction. These properties are typically quantified through the use of U-values – a measure of thermal transmittance or heat loss. The lower the U-value, the better the material insulates. In this article, we’ll dive into various substrates including park homes, Insulated Concrete Formwork (ICF), brickwork, blockwork, timber frame, Structurally Insulated Panels (SIPs), render carrier board, and stainless steel construction, discussing their U-values and impact on energy efficiency.

Park Home

Park homes are lightweight and mobile residential structures. The walls are often constructed using a method of sandwich panels, with insulative materials placed between layers of weather-resistant cladding. This design makes them affordable, flexible, and relatively energy-efficient. The U-values can vary depending on the thickness and type of insulation used. Some modern park homes can achieve U-values as low as 0.22 W/m²K, which surpasses many traditional residential buildings.

Insulated Concrete Formwork (ICF)

ICF is a system of formwork for reinforced concrete that stays in place as a permanent interior and exterior substrate for walls, floors, and roofs. The forms are interlocking modular units that are dry-stacked and filled with concrete. ICFs are recognised for their thermal efficiency, as the foam provides effective insulation while the concrete ensures excellent thermal mass. This combination results in a high-performance insulation solution. With U-values typically around 0.20 W/m²K, ICFs are an excellent choice for building energy-efficient homes.


Traditional brickwork involves the use of fired clay bricks and mortar. It’s one of the most common methods of construction due to its strength and durability. However, bricks themselves aren’t particularly good insulators. That’s why brickwork often includes cavity walls, with an insulation layer within the cavity to improve the overall U-value. Without additional insulation, a typical brick wall can have a U-value of around 2.0 W/m²K. This value is improved with cavity wall insulation which brings it down to approximately 0.30 W/m²K.


Blockwork usually involves the use of concrete or cement blocks. As with brickwork, the blocks alone do not insulate particularly well. But with the use of cavity insulation or insulative blocks, the thermal performance can be significantly improved. This method is robust and cost-effective. A single skin blockwork wall might have a U-value around 2.1 W/m²K. However, incorporating an insulated cavity can reduce the U-value considerably, similar to brickwork.

Timber Frame

Timber frame construction involves a structural timber frame that forms the load-bearing part of the building. These frames are typically then filled with insulation and enclosed with structural sheathing. Timber has a natural insulative property, and the use of additional insulation materials helps achieve impressive U-values. Additionally, it is a sustainable and renewable resource, making it an environmentally friendly choice. When constructed with insulation in the cavities, a timber frame can reach U-values as low as 0.13 to 0.18 W/m²K, making it a good choice for energy-efficient construction.

Structurally Insulated Panels (SIPs)

SIPs are an innovative construction material that incorporates insulation into their design. They consist of an insulating foam core sandwiched between two structural facings. The manufacturing process ensures airtightness and minimal thermal bridging, leading to excellent insulative properties. SIPs have a quick and easy installation process. The continuous insulation and minimal thermal bridging allow SIPs to achieve impressive U-values, often around 0.17 W/m²K.

Render Carrier Board

Render carrier boards provide a substrate for external render systems. These boards themselves offer some degree of insulation, but they are more often used in conjunction with insulation layers. Render carrier boards are popular due to their ease of installation and compatibility with a range of finishes and external cladding solutions. However, as they are relatively thin and not primarily intended as insulation, their standalone U-value is high, often above 1.5 W/m²K. They should ideally be used in combination with other insulative materials to reduce overall U-values.

Stainless Steel Construction

Stainless steel is for structural elements in commercial and industrial buildings. It offers high strength, durability, and resistance to corrosion but lacks thermal insulating properties. For these reasons, when stainless steel is used, it is crucial to incorporate a high-performance insulation layer in the design to ensure energy efficiency. However, it’s not a good insulator, with U-values typically above 5.0 W/m²K. It’s crucial to incorporate insulation when using stainless steel in construction to enhance energy efficiency.

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